1. Field of the Invention
The present invention relates to a linear solenoid valve apparatus and a method for assembling in which a hydraulic fluid flow is varied in accordance with an electric signal, for example, the valve of electric current and in particular relates to a linear solenoid valve used for controlling a primary regulator valve to regulate a line pressure.
2. The Prior Art
In general, a linear solenoid valve includes a modulator valve and an electromagnetic valve. The electromagnetic valve includes a plunger operated in accordance with an electric current. The modulator valve includes a spool, a spring biasing the spool, a feedback oil pressure chamber for moving the spool to a position supplying a modulated fluid pressure from a supply port to an output port. It is necessary to form a structure which supplies a modulated fluid pressure corresponding to an electric current in a fixed error range.
In answer to the above problem, as shown in FIG. 6, the value of the energizing force in an electromagnetic valve was predetermined (so-called flat characteristic) to be a constant, regardless of the manufacturing error for the plunger stroke in the pressure modulating range. It was necessary to set the tolerance between the core and the plunger accurately and very important to provide the proper magnetic pull in the electromagnetic valve for the so-called flat characteristic. The above-described tolerance, however, is limited to a fixed value when the product is manufactured and a manufacturing error in the size of the electromagnetic portion has a great influence on the so-called flat characteristic. In particular, high accuracy is necessary in the design of a linear solenoid valve apparatus disposed in a valve body and formed in a more compact size, as used in an automatic transmission installed in a restricted vehicle space. It is very difficult to provide so-called flat characteristic in a linear solenoid valve apparatus which has an electromagnetic valve formed of a short length (for example, in the case that the radial size of 31 o is shortened to 28 o) while keeping a high electric current (for example, 1A) to provide sufficient force, or even if a low electric current were used.
A linear solenoid valve having a plunged adjusted at a return position by a screw in an electromagnetic valve or adjusted at an axially fixed location by disposing an E-ring between the spool and the push rod is disclosed in Tokkaihei 2-180390 patent specification. Even with this structure, a small size solenoid valve apparatus cannot be designed in the tolerance range by modifying the above-described spool as shown by dotted line "a" in FIG. 6, while manufacturing high electric current, for example 1A.